Structures in contact with seawater, for example ships, oceanic constructions, fish farming nets, buoys and industrial water systems are constantly exposed to water inhabited by various organisms. Therefore, as time passes by, microorganisms such as bacteria and diatoms and, further, fouling organisms of larger size, for example such animals and plants as barnacles, mussels and sea lettuce, adhere to said structures. When the surfaces of the structures in contact with seawater are covered with such marine organisms, there take place corrosion of the covered portions, decreased marine fuel efficiency due to increased frictional resistance of the ship bottom against seawater, massive deaths of fish and shellfish due to clogging of fish farming nets, sinking of buoys due to reduced buoyancy.
To prevent these hazardous organisms from adhering, antifouling coatings have so far been applied. Typical of such antifouling coatings are those of the hydrolyzable type which contain a trialkyltin-containing polymer as an essential antifouling component. These antifouling coatings produce their effects when the trialkyltin-containing polymer is hydrolyzed in the weakly alkaline seawater environment, whereby the organotin compound is eluted and at the same time the coating vehicle becomes soluble in water, allowing elution of the antifoulant incorporated. However, the organotin compound eluted is highly toxic and harmful to the ecosystem. Ecofriendly antifouling means have therefore been required from the environmental pollution viewpoint.
Silicone rubber coating films are well known to be capable of producing an antifouling effect without elution of an antifoulant and/or some other component. The silicone rubber coating films have both water-repelling property and elasticity, together with other properties. These properties are utilized to produce the antifouling effect. However, such coating films are attacked by microorganisms in a very early phase following submersion thereof in seawater, with the result that they lose water-repelling property and allow rapid progress of adhesion of marine organisms.
Various proposals have been made to maintain water repellency and attain an improved lasting antifouling effect. Thus, for instance, Japanese Kokai publication She-53-79980 discloses a method which comprises incorporating a silicon- and metal-free organic compound, such as liquid paraffin, into a vulcanized silicone rubber. Japanese Kokoku Publication Sho-56-26272 discloses a method which comprises incorporating a silicone oil with a molecular weight of about 2,000 to 30,000 into a silicone rubber. Japanese Kokoku Publication Sho-60-3433 discloses a method which comprises adding a petroleum fraction-derived substance with a low critical surface tension, such as vaseline, to a silicone rubber. Japanese Kokai Publication Sho-54-26826 and Japanese Kokoku Publication Sho-57-16868 disclose a method which comprises incorporating various thermoplastic resins or a polyvinylbutyral resin into a silicone rubber. However, by these technologies, it is difficult to maintain the antifouling effect over a long period of time.
Japanese Kokai Publication Hei-07-328523 discloses a water-repelling coating film which comprises a resin coating film and minute particles at least the surface of which is hydrophobic and which have a mean size of 1 nm to 1 mm and are fixed on at least 20% of the coating film surface area. However, this technology still has a problem. Since the minute particles are caused to adhere to the resin in the course of coating film formation when the resin is still in the uncured or semicured state, the coating film, when used in water for a long cumulative period, allows the minute particles to come from it, whereby the water repellency of the coating film is impaired.
Therefore, there has been proposed an antifouling technology which comprises coating a substrate surface with a nonelution type coating composition comprising at least two kind of resins mutually incompatible with each other and insoluble in seawater and causing the coating composition to cure in situ to thereby form on the surface a microheterogenous coating film having a plurality of minute protrusions of silicone-grafted acrylic resin. In accordance with this technology, the size and fractional area of said minute protrusions are controlled so as to provide a coating film surface to which marine organisms can hardly attach themselves.
However, even to such a deliberately prepared coating film surface, marine organisms attach themselves after long exposure to seawater. Furthermore, unless the film structure formed is flawless or if it is impaired by chance not to mention an antifouling structure, the antifouling efficiency is lost.